The phase transition kinetics of temperature-sensitive N-isopropylacrylamide hydrogel was investigated. It was indicated that the apparent second order kinetic equations describe its swelling and collapse behavior quite well. Its swelling kineticequation is (dR)/(dt)= ks(Rg- R)2 and the collapse one is- (dR)/(dt)= (kc(R-Rc)2. A para-meter of "reasonable time of separating circle per swelling ratio" was introduced for describing the separating behavior of polymer water solution. It can be calculated from initial swelling ratio

equilibrium swelling ratio

swelling kinetic constant and collapse kinetic constant. The equation is At the ideal case

total time for separation is proportional to The ratio factor is equal to the total water weight separated divided by the dry hydrogel weight

that is When the calculated value of the total time based on the two ki-netic equations was located into the region of it means that the dry hydrogel weight chosen and the circular swelling ratio region chosen are reasonable.